1. Field of the Invention
The present invention relates to image pickup apparatus and, more particularly, to an apparatus which is suitably used in a so-called multi-element image pickup device which picks up an object image using a plurality of image pickup elements.
2. Related Background Art
A conventional video camera reproduces a picked-up image on a television screen. For this reason, the recording rate of the video camera is determined to be a predetermined value. For example, in the case of the NTSC system, 60 fields (frames) of images are recorded per second. More specifically, an exposure is performed in 1/60 sec, and an image signal is read out and recorded in the next 1/60 sec.
On the other hand, upon detection of the state of incident light for auto-focus (to be abbreviated as AF hereinafter) control, auto-exposure (to be abbreviated as AE hereinafter) control, and the like, a signal for a photographing operation is read out, and thereafter, required data is fed back from this signal to control the iris and the lens driving operation.
For example, photometry for the AE control is performed based on a signal obtained by averaging, by an integrator, a plurality of frames of (three to four frames) of signals, which are output from an image pickup element and are amplified by an amplifier. On the other hand, in the AF control, a high-frequency component of a signal of each field is extracted, and the lens is controlled to maximize the extracted component.
Japanese Laid-Open Patent Application No. 4-212577 discloses a technique for performing two or more AF scans per field period by setting the read-out range in the vertical direction to be a portion of a photographing screen including a distance measurement frame.
As described above, the video camera can control the iris and lens only after a signal exposed in one field period (1/60 sec) is read out for one field period. For this reason, since the video camera cannot quickly respond to an abrupt movement of an object or an abrupt change in brightness, the photographed image often becomes too dark or bright.
For the same reason as above, the focal point cannot often be adjusted to an object which moves abruptly. When a photographing light beam is split by, e.g., a prism, and is received by AE and AF sensors different from the image pickup element, the amount of light guided to the image pickup element for a photographing operation decreases, resulting in deterioration of image quality and an increase in cost.
In the technique proposed by Japanese Laid-Open Patent Application No. 4-212577, since a signal read out from the image pickup element corresponds to a portion of the screen, the signal cannot be used in a photographing operation. Therefore, in this case, since a photographing operation is started after the end of the AF operation, an image to be photographed cannot be confirmed during the AF operation, and the time required until the photographing operation is started is long, resulting in a large time lag.